Islamabad- Five people were killed and at least a dozen others injured in a gas cylinder explosion at a scrap yard in the eastern city of Lahore on Thursday, police and rescue officials said.

The blast, which occurred in the congested downtown market of Misri Shah, caused the galvanized steel roof of the building to collapse and trap several workers inside.

"So far we can confirm five casualties in the explosion that was caused by accident," local police chief Malik Iqbal said.

Rescuers struggled to pull out the victims but faced difficulty in lifting the heavy steel sheets in the absence of cranes.

Rescue service chief Rizwan Naseer said paramedics took 12 people to the hospital, adding that at least three of the victims were in a critical condition.

According to Iqbal, workers were dismantling steel using gas cutters when a pressure build-up caused the cylinders to explode. He ruled out sabotage.

Scores of scrap dealers across the country receive discarded metal, including dud ammunition and casing from neighbouring Afghanistan and Iran, which sometimes explode while being processed.